1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a pouch type secondary battery having a pouch case with an improved structure.
2. Description of the Related Art
Secondary batteries are rechargeable. They can be made small and have a large capacity. Representative secondary batteries are nickel-metal hydride (Ni-MH) batteries and lithium (Li) secondary batteries.
According to the type of a case accommodating an electrode assembly, secondary batteries can be divided into cylindrical batteries using a cylindrical aluminum can, prismatic batteries using a prismatic aluminum can, and pouch type batteries using a thin pouch case.
Pouch type secondary batteries are manufactured by housing an electrode assembly generating a predetermined current and liquid electrolyte into a pouch case made of a flexible material in a bag shape and hermetically sealing the pouch case. Such a pouch type secondary battery is light and flexible to facilitate the design of a packaging structure. Accordingly, the pouch type secondary batteries are widely used for portable telephones and video cameras. FIG. 1 illustrates a pouch type secondary battery to which a protective circuit module is coupled. First and second electrode tabs 2a and 2b extend from and are folded on a sealing portion 3a at an end of a pouch case 1 having a bag shape. A protective circuit board 4 is coupled to the first and second electrode tabs 2a and 2b. Sealing portions 3b are folded one or two times and respectively closely stuck to opposite sides of the pouch case 1.
However, since a corner 3c of the sealing portions 3a and 3b protrudes outward, the corner 3c may interfere with an external pack used for packaging. In particular, since such an external pack tends to curve, the corner 3c may be crushed by the curved external pack.
The pouch case 1 is usually made of a metal foil such as an aluminum foil, a surface of which underwent an insulation process. In other words, modified polypropylene, i.e., a polymer resin, for example, cast polypropylene (CPP), is deposited on the aluminum foil, thereby forming a thermal adhesive layer, and a resin material such as nylon or polyethylene terephtalate (PET), is formed at the outer side of the aluminum foil. However, as shown in FIG. 1, cutting is performed at a portion of the pouch case 1 where the electrode tabs 2a and 2b are exposed outside. Accordingly, a material of the pouch case 1, and particularly, a core material, i.e., the aluminum foil, may be exposed outside of the cut portion of the pouch case 1.
In particular, the corner 3c may contact a circuit pattern (not shown) of the protective circuit board 4 or electronic parts (not shown) mounted on the protective circuit board 4. This contact may cause an electric short circuit of a module of the protective circuit board 4 and also accelerate the corrosion of the corner 3c. 
These problems occur more frequently when the corner 3c is crushed by an external pack. In other words, a cut end of the crushed corner 3c is more likely to contact the circuit pattern of the protective circuit board 4 or the electronic parts.
Japanese Patent Publication No. 2000-268807 discloses a secondary battery having a structure in which two corners at opposite ends of a sealing portion from which electrode tabs extend are cut away. This secondary battery includes cut portions C at the opposite corners of a sealing portion 3a from which electrode tabs 2a and 2b extend, as shown in FIGS. 2B and 2C. Sealing portions 3b at the opposite sides of the secondary battery are folded two times along the folding lines L1 and L2 to increase a size K of a space for accommodating a protective circuit board 4, as shown in FIGS. 2A and 2D. In such a secondary battery, the cut ends of the cut portions C are disposed at the sides of the protective circuit board 4. As a result, the cut ends contact the protective circuit board 4 and thus may electrically short circuit a protective circuit module of the protective circuit board 4. In other words, this second battery cannot overcome the problems described above with reference to FIG. 1.